Shock absorbing material

ABSTRACT

A shock-absorbing material composed of an expansion-molded article of polypropylene resin particles is excellent in shock-absorbing property and impact resilience compared with shock-absorbing materials composed of other resin materials, but has not been said to be satisfactory in stiffness and energy absorption efficiency. The present invention relates to a shock-absorbing material composed of an expansion-molded article produced by using foamed particles comprising, as a base resin, a polypropylene homopolymer obtained by using a metallocene polymerization catalyst. The base resin has a tensile modulus of at least 15,000 kgf/cm 2 , and the expansion-molded article has a crystal structure that an inherent peak a and a high-temperature peak b appear as endothermic peaks on a DSC curve obtained by the differential scanning calorimetry of the molded article. The high-temperature peak b is a peak appeared on the temperature side higher than a temperature corresponding to the inherent peak a, and the quantity of heat at the high-temperature peak is at least 25 J/g.

This application is a 371 of PCT/JP97/02792 filed on Aug. 8, 1997 whichclaims priority from Japanese Patent Application No. HEI8-229289 FiledAug. 12, 1996.

TECHNICAL FIELD

The present invention relates to a shock-absorbing material suitable foruse in a core material for automotive bumpers, or the like.

BACKGROUND ART

At present, shock-absorbing materials used as core materials forautomotive bumpers, and the like are made mainly of a synthetic resinfoam. An automotive bumper making use of a synthetic resin foam isgenerally composed of a core formed of the synthetic resin foam, and askin material made of a synthetic resin, with which the core is covered.

Shock-absorbing materials used as core materials for the automotivebumpers, and the like are generally required to satisfy 1) to haveexcellent energy absorption performance, 2) to have an excellentdimensional recovery factor and 3) to provide a low-density andlight-weight material at the same time.

Japanese Patent Application Laid-Open Nos. 221745/1983 and 189660/1985each disclose core materials for automotive bumpers, which satisfies theabove three conditions.

These publications disclose polypropylene and ethylene-propylenecopolymers as base resins for bumper cores.

Further, Japanese Patent Application Laid-Open No. 158441/1990 andJapanese Patent Application Laid-Open No. 258455/1995 disclose1-butene-propylene random copolymers, and 1-butene-propylene randomcopolymers and ethylene-1-butene-propylene random terpolymers,respectively, as materials used for bumper cores.

When a shock-absorbing material used as a bumper core or the like isproduced by using a polypropylene resin as a material, there isgenerally used the so-called bead molding process in which foamedparticles are filled into a mold and heated to expand the foamedparticles, thereby mutually fusion-bonding them to obtain anexpansion-molded article conforming to the mold. The expansion-moldedarticle of the polypropylene resin particles obtained by this process isexcellent in shock-absorbing property and impact resilience and hasexcellent physical properties such as light weight and small residualstrain.

Accordingly, the shock-absorbing material composed of theexpansion-molded article of the polypropylene resin particles hasexcellent properties compared with shock-absorbing materials composed ofother resin materials. However, its stiffness and energy absorptionefficiency are not necessarily satisfactory, and so it yet leaves roomto improve. Raw resins actually used for producing shock-absorbingmaterials at present are propylene copolymers. A polypropylenehomopolymer itself is a high-stiffness polymer and most suitable for useas a raw resin for producing a shock-absorbing material. On the otherhand, such a resin has involved problems that its molding temperaturebecomes high due to its high melting point, and the molding temperaturerange for successfully expanding it is limited due to its viscoelasticproperty. As described above, the use of the polypropylene homopolymeras the molding resin has involved the difficulty of presetting theoptimum conditions upon molding and hence a problem that defectivefusion bonding among resin particles is caused by, for example, a slighterror in temperature setting. Therefore, this resin has been poor inmoldability.

The reason why the propylene copolymers are actually used as moldingresins is that their moldability is better compared with thepolypropylene homopolymer.

However, the propylene copolymers naturally has the demerit that theirstiffness is low, and so some attempts have been made to improve thestiffness by lessening the content of other component(s) than propylenein the copolymers. However, only unsatisfactory results have beenobtained. In addition, any shock-absorbing material comprising apropylene copolymer as a base resin is unsatisfactory even from theviewpoint of energy absorption efficiency.

The investigation by the present inventors revealed that when apolypropylene homopolymer (hereinafter referred to as “the metallocenePP”) obtained by using a metallocene polymerization catalyst is used asa molding resin, a shock-absorbing material having good physicalproperties is obtained.

However, it was also confirmed that in order for a shock-absorbingmaterial to achieve high stiffness and energy absorption efficiency, themere use of such a polypropylene homopolymer as a base resin isinsufficient, and other factors than this must be added. Thus thepresent inventors have carried out a further investigation. As a result,it has been found that when the tensile modulus of the metallocene PPand the quantity of heat at a high-temperature peak appeared on a DSCcurve obtained by the differential scanning calorimetry of the resultingmolded article are defined within specific numerical ranges, ashock-absorbing material having high stiffness and energy absorptionefficiency can be obtained. The present invention has been led tocompletion on the basis of this finding.

It is an object of the present invention to provide a shock-absorbingmaterial used as a bumper core or the like, which is produced by using apolypropylene homopolymer as a molding raw resin.

Another object of the present invention is to provide a shock-absorbingmaterial excellent in stiffness and energy absorption efficiencycompared with the conventional materials.

A further object of the present invention is to provide ashock-absorbing material which imparts the advantage in productionconditions that the pressure of steam fed into a mold upon molding canbe controlled low.

DISCLOSURE OF THE INVENTION

The present invention relates to a shock-absorbing material composed ofan expansion-molded article produced by using foamed particlescomprising a metallocene PP as a base resin, wherein the base resin hasa tensile modulus of at least 15,000 kgf/cm², and the expansion-moldedarticle has a crystal structure that an inherent peak and ahigh-temperature peak appear as endothermic peaks on a DSC curveobtained by the differential scanning calorimetry of the molded article.The term “high-temperature peak” as used herein means a peak appeared onthe temperature side higher than a temperature corresponding to theinherent peak of endothermic peaks appeared on a DSC curve obtained byheating 2 to 4 mg of a specimen cut out of the molded article to 220° C.at a heating rate of 10° C./min by means of a differential scanningcalorimeter.

The foamed particles of the metallocene PP used in the production of theshock-absorbing material according to the present invention are thosehaving a crystal structure that an inherent peak and a high-temperaturepeak appear as endothermic peaks on a DSC curve obtained by thedifferential scanning calorimetry of the foamed particles, and aquantity of heat of at least 25 J/g at the high-temperature peak.

When molding is conducted by using such foamed particles, theabove-described crystal structure does not disappear, and so a moldedarticle produced also has a similar crystal structure that an inherentpeak and a high-temperature peak appear as endothermic peaks on its DSCcurve. Further, the quantity of heat at the high-temperature peak in themolded article also indicates almost the same value as the quantity ofheat at the high-temperature peak in the foamed particles, and itsnumerical value is at least 25 J/g.

The shock-absorbing material according to the present invention featuresthat it is composed of an expansion-molded article of foamed particlescomprising, as a base resin, a polypropylene homopolymer obtained byusing a metallocene polymerization catalyst, the base resin has atensile modulus of at least 15,000 kgf/cm², the expansion-molded articlehas a crystal structure that an inherent peak and a high-temperaturepeak appear as endothermic peaks on a DSC curve obtained by thedifferential scanning calorimetry of the molded article, and thequantity of heat at the high-temperature peak in the molded article isat least 25 J/g. The fact that the shock-absorbing material has thesethree factors brings about an effect of markedly improving stiffness andenergy absorption efficiency compared with the shock-absorbing materialscomposed of an expansion-molded article comprising the conventionalpolypropylene copolymer as a base resin.

The expansion molded article has compression stress of at least 7.28kgf/cm² and an energy absorption efficiency of at least 72.4 as reportedin the below Table 2, and as illustrated in FIG. 3A and in FIG. 3B,respectively.

When the polypropylene homopolymer (metallocene PP) obtained by usingthe metallocene polymerization catalyst and used as a raw material forproducing the shock-absorbing material according to the presentinvention is compared with a polypropylene homopolymer obtained by usinga Ziegler-Natta catalyst, the metallocene PP characteristically has alower melting point when both polymers have the same tensile modulus. Asa result, when molding is conducted by using foamed particles comprisingthe metallocene PP as a base resin, the pressure of steam fed into amold can be controlled lower, so that the following effects can bebrought about. Namely, the consumption of heat can be decreased toreduce production cost, and the durability of the mold can be enhanced.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary chart of a first DSC curve of a moldedarticle;

FIG. 2 illustrates an exemplary chart of a second DSC curve of themolded article;

FIG. 3A diagrammatically illustrates the relationship between thequantity of heat at the high-temperature peak of a molded article andcompression stress; the FIG. 3B diagrammatically illustrates therelationship between the quantity of heat at the high-temperature peakof a molded article and energy absorption efficiency (%).

FIG. 4 illustrates an exemplary chart of a DSC curve of a base resin;and

FIG. 5 illustrates a stress-strain diagram of a molded article.

BEST MODE FOR CARRYING OUT THE INVENTION

The present invention relates to a shock-absorbing material, such as anautomotive bumper core, composed of an expansion-molded article producedby using foamed particles comprising, as a base resin, a polypropylenehomopolymer (metallocene PP) obtained by using a metallocenepolymerization catalyst. The base resin of the foamed particles may becomposed of either only the polypropylene homopolymer, or a blend of thepolypropylene homopolymer as a main component with another resin orelastomer as will be described subsequently.

The metallocene polymerization catalyst used in obtaining themetallocene PP in the present invention is composed of a transitionmetal complex component having a metallocene structure, and a promotorcomponent such as an alumoxane, Lewis acid or ionic compound.

The transition metal complex component has a structure that 2 conjugated5-membered ring groups C₅H_(4−m)R¹ _(m) and C₅H_(4−n)R² _(n) arecrosslinked by a crosslinking group Q, and transition metal compoundMeXY is coordinated with this crosslinked product, and is represented bythe following general formula (1):

Q(C₅H_(4−m)R¹ _(m))(C₅H_(4−n)R² _(n))MeXY  (1)

wherein m and n are integers satisfying 0≦m, n≦4.

The conjugated 5-membered ring groups C₅H_(4−m)R¹ _(m) and C₅H_(4−n)R²_(n) may be the same or different from each other. When m (or n) is 0,the conjugated 5-membered ring group is a cyclopentadienyl group.

R¹ and R² substituted on the respective conjugated 5-membered ringgroups may be the same or different from each other. When one of theconjugated 5-membered ring groups has a plurality of the substituents R¹(or R²), these substituents R¹ (or R²) may be the same or different fromeach other.

Examples of the substituents R¹ and R² include hydrocarbon groups,halogen atoms and alkoxy groups. The hydrocarbon groups may contain asilicon, phosphorus, boron or the like. The hydrocarbon groups may bemonovalent substituents, or divalent substituents which are each bondedto the conjugated 5-membered ring group to form a ring. Therefore,indenyl and fluorenyl groups which are fused rings formed by sharing adouble bond with the conjugated 5-membered ring group such as acyclo-pentadienyl group may also be included in the concept of theconjugated 5-membered ring groups referred to in the present invention.

Examples of the crosslinking group Q crosslinking the two conjugated5-membered ring groups include alkylene groups such as methylene,ethylene, isopropylene, phenylmethylene and diphenylmethylene groups,cycloalkylene groups such as a cyclohexylene group, silylene groups suchas silylene, dimethylsilylene, phenylmethylsilylene, diphenylsilylene,disilylene and tetramethyldisilylene groups, and hydrocarbon groupscontaining germanium, phosphorus, boron or aluminum.

Me in the transition metal compound MeXY means a transition metal ofGroup IVB to VIB in the periodic table and is preferably titanium,zirconium or hafnium. X and Y combined with the transition metal Me areindependently hydrogen, halogen, or a hydrocarbon, alkoxyl, amino oralkylamino group. The hydrocarbon group may contain phosphorus orsilicon. X and Y may be the same or different from each other.

Specific example of such a transition metal complex component includecompounds with conjugated 5-membered ring groups crosslinked by analkylene group, such as ethylenebis(2-methylindenyl)zirconiumdichloride, ethylenebis(2-methyl-4,5,6,7-tetrahydroindenyl)zirconiumdichloride, ethylenebis(2,4-dimethylindenyl)zirconium dichloride,ethylenebis(2,4-dimethyl-4-hydroazurenyl)-zirconium dichloride,ethylenebis(4,5,6,7-tetrahydro-indenyl)hafnium chloride, and compoundswith conjugated 5-membered ring groups crosslinked by a silylene group,such as dimethylsilylenebis(4,5,6,7-tetrahydroindenyl)zirconiumdichloride, dimethylsilylenebis(2-methylindenyl)zirconium dichloride,dimethylsilylenebis(2-methyl-4,5,6,7-tetrahydroindenyl)zirconiumdichloride, dimethylsilylene-bis(2,4-dimethylindenyl)zirconium chlorideand dimethyl-silylenebis(2,4-dimethyl-4-hydroazurenyl)zirconiumdichloride.

On the other hand, specific examples of the promotor component includealumoxanes such as methylalumoxane, isobutylalumoxane andmethylisobutylalmoxane, Lewis acids such as triphenylboron,tris(pentafluorophenyl)boron and magnesium chloride, and ionic compoundssuch as diemthylanilinium tetrakis(pentafluorophenyl)boron andtriphenylcarbinium tetrakis(pentafluorophenyl)boron. These promotorcomponents may be used in combination with an organoaluminum compoundsuch as trimethylaluminum, triethylaluminum or truisobutylaluminum.

The shock-absorbing material according to the present invention isobtained by filling the foamed particles comprising the metallocene PPas a base resin into a mold which can be closed but not be sealedhermetically and has a desired shape, and feeding steam into the mold toheat and expand the foamed particles, thereby mutually fusion-bondingthe foamed particles to obtain an expansion-molded article conforming tothe mold.

The base resin constituting the shock-absorbing material according tothe present invention is required to have a tensile modulus of at least15,000 kgf/cm², preferably at least 15,500 kgf /cm². If the tensilemodulus is lower than 15,000 kgf/cm² ₁ it cannot be expected that thestiffness and energy absorption efficiency of the resultingshock-absorbing material are markedly improved.

When the tensile modulus is measured, the measurement may be conductedby using a specimen cut out of a sheet obtained by melting pellets ofthe base resin into the sheet. However, it is actually more convenientto make a specimen from an expansion-molded article and conduct themeasurement using this specimen. The following method can be adopted asa measuring method of the tensile modulus in the present invention.

Namely, a specimen is cut out of an expansion-molded article sample, andheated and pressed for 10 minutes by a hot press controlled to 220° C.,thereby melting and deaerating the specimen to form a sheet having athickness of 1 mm±0.1 mm. The sheet thus obtained is used to measure itstensile modulus in accordance with JIS K 7113 under the followingconditions:

Specimen: JIS No. 2 type Testing rate: 50 mm/min Distance betweenchucks: 80 mm.

When a metallocene PP having a tensile modulus of at least 15,000kgf/cm² (preferably at least 15,500 kgf/cm²) is used as a base resin, itgoes without saying that the tensile modulus value can be controlledwithin the above range.

The tensile modulus value of a metallocene PP generally varies accordingto the crystallinity, average molecular weight and molecular weightdistribution of the metallocene PP, the kind of a polymerizationcatalyst used, and the like. Accordingly, if numerical values of theseparameters are suitably selected upon the production of a metallocenePP, the tensile modulus of the resulting metallocene PP can becontrolled within the above range.

Preferable examples of the metallocene PP suitable for use in theproduction of the shock-absorbing material according to the presentinvention include those marketed under the trade names of “Achieve3844”, “Achieve 3825” and “Achieve 3904” by Exxon Co. in U. S. A.

The DSC curve obtained by the differential scanning calorimetry of amolded article in the present invention means a DSC curve obtained byheating 2 to 4 mg of a specimen cut out of an expansion-molded articleconstituting the shock-absorbing material according to the presentinvention to 220° C. at a heating rate of 10° C./min by means of adifferential scanning calorimeter.

Here, the DSC curve obtained by heating the above specimen from roomtemperature to 220° C. at a heating rate of 10° C./min is referred to asa first DSC curve (illustrating an example thereof in FIG. 1), and a DSCcurve obtained by cooling the specimen from 220° C. to about 40° C. at acooling rate of 10° C./min and heating it again to 220° C. at a heatingrate of 10° C./min is referred to as a second DSC curve (illustrating anexample thereof in FIG. 2). In this case, as illustrated in FIG. 1,endothermic peaks a and b arise on the first DSC curve. Of theseendothermic peaks, the endothermic peak b appeared on the temperatureside higher than a temperature corresponding to the endothermic peak aappears only on the first DSC curve, and does not appear on the secondDSC curve.

The endothermic peak a appeared on both first and second DSC curves isattributable to the absorption of heat upon the so-called fusion of themetallocene PP which is a base resin, and is an endothermic peakinherent in the metallocene PP. The endothermic peaks a and b willhereinafter be referred to as an inherent peak and a high-temperaturepeak, respectively.

The high-temperature peak b appeared only on the first DSC curve isattributable to the existence of a crystal structure different from thecrystal structure of an expansion-molded article on the DSC curve ofwhich no high-temperature peak b appears.

More specifically, since the inherent peak a appears on both first andsecond DSC curves, whereas the high-temperature peak b appears only onthe first DSC curve, and does not appear on the second DSC curveobtained by heating the specimen under the same conditions, it isconsidered that the crystal structure of the expansion-molded article,in which the high-temperature peak b appears together with the inherentpeak a, is not attributable to the crystal structure of the base resinitself, but attributable to a crystal structure inherent in theexpansion-molded article as a result of having gone through heathistory.

Incidentally, it is desired that a difference between the temperaturecorresponding to the top of the high-temperature peak b appeared on thefirst DSC curve and the temperature corresponding to the top of theinherent peak a appeared on the second DSC curve be greater, and thedifference between them is at least 50° C., preferably at least 10° C.In FIG. 1, the two endothermic peaks are drawn by a gently-slopingcurve. However, the DSC curve does not always become such agently-sloping curve, and so in some cases, a plurality of overlappingendothermic peaks may appear on a chart, and 2 endothermic peaks of theinherent peak and high-temperature peak may appear on the chart as awhole.

The high-temperature peak b is confirmed by the comparison of the firstDSC curve with the second DSC curve. The quantity of heat at thehigh-temperature peak b is determined by the following method. As shownin FIG. 1, a straight line is first drawn between a point α at 80° C. onthe DSC curve, and a point β on the DSC curve which indicates themelting completion temperature of the base resin. A line parallel to theordinate axis of the graph is then drawn from a point γ on the DSC curvewhich corresponds to a valley between the inherent peak a and thehigh-temperature peak b, to the straight line connecting the point a andthe point β. The intersection thereof is regarded as a point δ. Thequantity of heat corresponding to a section (an area hatched in FIG. 1)surrounded by a straight line connecting the point δ thus obtained andthe point β, the straight line connecting the point γ and the point δ,and a DSC curve connecting the point γ and the point β is determined asthe quantity of heat at the high temperature peak b.

The quantity of heat (hereinafter referred to as the quantity of heat atthe high-temperature peak) at the high-temperature peak b in theexpansion-molded article constituting the shock-absorbing materialaccording to the present invention is at least 25 J/g, preferably atleast 27 J/g, more preferably at least 30 J/g.

In the present invention, a shock-absorbing material used as a bumpercore or the like having high stiffness and excellent energy absorptionefficiency can be provided by molding foamed particles comprising themetallocene PP as a base resin into an expansion-molded article andcontrolling the tensile modulus of the base resin and the quantity ofheat at the high-temperature peak of the molded article to at least15,000 kgf/cm² and at least 25 J/g, respectively. However, if thequantity of heat at the high-temperature peak is lower than 25 J/g, thestiffness inherent in the base resin cannot be derived, and so even ifthe tensile modulus of the base resin is 15,000 kgf/cm² or higher, itcannot be expected to provide a shock-absorbing material markedlyimproved in stiffness and energy absorption efficiency.

A curve A shown in FIG. 3A is an example diagrammatically illustratingthe relationship between the quantity of heat at the high-temperaturepeak of an expansion-molded article constituting the shock-absorbingmaterial according to the present invention and stress under 50%compression. As also understood from this example, the compressionstress of the molded article is markedly lowered if the quantity of heatat the high-temperature peak is lower than 25 J/g, and so the stiffnessinherent in the base resin cannot be derived.

The curve A is a curve about an expansion-molded article having adensity of 0.06 g/cm³ produced by using a metallocene PP (trade name:“Achieve 3844”; product of Exxon Co. in U. S. A.) having a tensilemodulus of 22,000 kgf/cm² and a melting point T_(m) of 150° C. as a baseresin. A curve B in FIG. 3 is a curve about an expansion-molded articlehaving a density of 0.06 g/cm³ produced by using a propylene-ethylenerandom copolymer (ethylene content: 2.3% by weight) having a tensilemodulus of 12,000 kgf/cm² and a melting point T_(m) of 146° C. as a baseresin. As shown by this curve B, the molded article comprising thepropylene copolymer as a base resin has no point of inflection relatedto the compression stress at 25 J/g. The compression stresscharacteristics having a point of inflection at 25 J/g arecharacteristic of the expansion-molded article comprising themetallocene PP as a base resin, which constitutes the shock-absorbingmaterial according to the present invention.

The curve A illustrated in FIG. 3 indicates that if the quantity of heatat the high-temperature peak of the metallocene PP is 25 J/g or higher,its compression stress becomes high, and so the stiffness of the resinalso becomes high. When the quantity of heat at the high-temperaturepeak is 25 J/g or higher, a change in compression stress is slight.Therefore, the degree of stiffness imparted to the shock-absorbingmaterial can be controlled with ease.

The FIG. 3B diagrammatically illustrates the relationship between thequantity of heat at the high-temperature peak of a molded article andthe energy absorption efficiency (%).

The curve C in FIG. 3B is an example diagrammatically illustrating therelationship between the quantity of heat at the high-temperature peakof an expansion-molded article constituting the shock-absorting materialand energy absorption efficiency (%) as reported in Table 2.

The expansion-molded article on the DSC curve of which ahigh-temperature peak b appears can be obtained by conducting thedifferential scanning calorimetry of foamed particles in the same manneras the above-described method for obtaining the DSC curves of the moldedarticle and using foamed particles on the first DSC curve of which aninherent peak and a high-temperature peak appear like the first DSCcurve of the molded article to mold them. Such foamed particles areobtained by defining the temperature and time at and for which particlesof the metallocene PP are heated and held before expansion, and furthera foaming temperature in a production process of foamed articles whichwill be described subsequently.

More specifically, when foamed particles are produced under the specificconditions, the foamed particles obtained under such conditions come tohave a crystal structure that a high-temperature peak appears on a DSCcurve thereof. Such a crystal structure does not disappear upon molding,and so an expansion-molded article obtained by the molding also has asimilar crystal structure. Incidentally, the quantity of heat at thehigh-temperature peak of the foamed particles can be determined from thefirst DSC curve of the foamed particles in accordance with the sameprocedure as that for obtaining the quantity of heat at thehigh-temperature peak of the molded article. The value thereof isapproximately equal to the quantity of heat at the high-temperature peakof the molded article.

More specifically, when foamed particles having a crystal structure thatan inherent peak and a high-temperature peak appear on a DSC curveobtained by the differential scanning calorimetry thereof andcomprising, as a base resin, a metallocene PP, and having a quantity ofheat of at least 25 J/g at the high-temperature peak are used to moldthem, an expansion-molded article having the same crystal structure andquantity of heat at the high-temperature peak as those of the foamedparticles can be obtained.

In the present invention, the metallocene PP is used as a base resin forfoamed particles. However, a mixture obtained by blending themetallocene PP as a main component with another resin or elastomer as asecondary component may be used as the base resin so far as nodetrimental influence is thereby imposed on the effects of the presentinvention.

Examples of another resin capable of being mixed with the metallocene PPinclude various thermoplastic resins, such as polypropylene resinsobtained by using another Ziegler-Natta catalyst than the metallocenepolymerization catalysts, polyolefin resins such as high densitypolyethylene, linear low density polyethylene, very low densitypolyethylene, low density polyethylene, ethylene-vinyl acetatecopolymers, ethylene-acrylic ester copolymers, ethylene-acrylic acidcopolymers and ethylene-methacrylic acid copolymers, and polystyreneresins.

Examples of the elastomer capable of being mixed with the metallocene PPinclude solid rubbers such as ethylene-propylene rubber,ethylene-i-butene rubber, propylene-1-butene rubber, styrene-butadienerubber and hydrogenated products thereof, isoprene rubber, neoprenerubber, and nitrile rubber, and polystyrene elastomers such asstyrene-butadiene block copolymer elastomers and hydrogenated productsthereof. Besides the above, various kinds of other elastomers may beused.

The mixing amount of the secondary component is less than 50 parts byweight, preferably less than 30 parts by weight per 100 parts by weightof the metallocene PP. Any mixing amount may be selected within theabove range of the mixing amount so far as the tensile modulus of theresulting base resin is retained in the above-described range, and thedevelopment of the high-temperature peak and the value of the quantityof heat at the high-temperature peak are not adversely affected.

Various kinds of additives may be incorporated into the base resin.Examples of the additives include antioxidants, ultraviolet absorbents,antistatic agents, flame retardants, metal deactivators, pigments, dyes,inorganic substances and nucleating agents. These additives are mixed inan amount of 20 parts by weight or less, preferably 5 parts by weight orless per 100 parts by weight of the base resin though it variesaccording to the physical properties required of the resultingexpansion-molded article. Incidentally, examples of the inorganicsubstances to be mixed with the base resin include talc, calciumcarbonate, borax and aluminum hydroxide.

The mixing of the above-described secondary component and additives withthe metallocene PP is generally conducted by melting and kneading them.For example, they are kneaded at a desired temperature by means of anyof various kneading machines such as a roll mixer, screw mixer, Banburymixer, kneader, blender and mill.

The foamed particles used for obtaining the shock- absorbing materialaccording to the present invention are produced by first of allproducing resin particles in the form of pellets by, for example, ameans in which the base resin is melted and kneaded in an extruder, themelt is extruded in the form of a strand, and the strand is cooled andthen chopped into proper lengths, or chopped into proper lengths andthen cooled. The resin particles thus produced are then dispersed in adispersion medium in the presence of a foaming agent in a closed vessel,and a dispersing agent is added to the dispersion medium as needed. Thecontents are then heated to a temperature of at least the softeningtemperature of the resin particles to impregnate the resin particleswith the foaming agent. Thereafter, the closed vessel is opened at oneend thereof to release the resin particles and the dispersion medium atthe same time from the vessel into an atmosphere of a pressure(generally, under atmospheric pressure) lower than the internal pressureof the vessel while maintaining the internal pressure of the vessel atthe vapor pressure of the foaming agent or higher, thereby expanding theresin particles to obtain foamed particles.

When foamed particles obtained by heating, upon the heating of the resinparticles in the closed vessel, the resin particles to an optionalheating temperature T_(a) within a range of from not lower than [themelting point T_(m) of the metallocene PP−15° C.] to lower than themelting completion temperature T_(e) of the metallocene PP withoutheating the resin particles to the melting completion temperature T_(e)of the metallocene PP or higher, holding them at this temperature T_(a)for a sufficient period of time (preferably, about 10 to 60 minutes),and then heating them to an optional heating temperature T_(b) within arange of from not lower than [the melting point T_(m)−150° C.] to [themelting completion temperature T_(e)+5° C.] to hold them at thistemperature T_(b) for an additional sufficient period of time(preferably, about 10 to 60 minutes) if necessary, thereby expanding theresin particles, are used, an expansion-molded article having a crystalstructure on the DSC curve of which a high-temperature peak b appearscan be obtained.

The quantity of heat at the high-temperature peak on the DSC curve ofthe molded article mainly depends on the holding temperatures T_(a) andholding time at the temperature T_(a), the holding temperature T_(b) andholding time at the temperature T_(b) for the resin particles upon theproduction of the foamed particles, and a heating rate.

Incidentally, the temperature ranges described above are temperatureranges in the case where an inorganic gas type foaming agent is used asthe foaming agent. Accordingly, when the foaming agent is changed to avolatile organic foaming agent, the suitable temperature ranges areshifted on the temperature sides lower than the above temperature rangesaccording to the kind and amount of the volatile organic foaming agentused.

The melting point T_(m) of the metallocene PP means a temperaturecorresponding to a top of an inherent peak a appeared on a second DSCcurve (illustrating an example thereof in FIG. 4) obtained by using 2 to4 mg of the metallocene PP as a sample to conduct its differentialscanning calorimetry in the same manner as the above-described methodfor obtaining the DSC curves of the expansion-molded article, while themelting completion temperature T_(e) means a temperature correspondingto a point on the temperature side higher than the temperaturecorresponding to the inherent peak a, at which the DSC curve is justreturned from the top of the inherent peak a to a position of the baseline.

The metallocene PP used in the production of the shock-absorbingmaterial according to the present invention preferably has a meltingpoint T_(m) of 140 to 160° C., most desirably 145 to 158° C.

As the foaming agent used in obtaining the foamed particles, there maybe generally used a volatile foaming agent, such as an aliphatichydrocarbon such as propane, butane, pentane, hexane or heptane, analicyclic hydrocarbon such as cyclobutane or cyclopentane, or ahalogenated hydrocarbon such as trichlorofluoromethane,dichlorodifluoromethane, dichlorotetrafluoroethane, methyl chloride,ethyl chloride or methylene chloride, an inorganic gas type foamingagent such as nitrogen, carbon dioxide, argon or air, or a mixerthereof. In particular, he inorganic gas type foaming agent is preferredin that it causes no ozonosphere destruction and is cheap, withnitrogen, air or carbon dioxide being particularly preferred.

The amount of the foaming agents other than nitrogen and air to be usedis generally 2 to 50 parts by weight per 100 parts by weight of theresin particles. When nitrogen or air is used as the foaming agent onthe other hand, it is introduced into a closed vessel under a pressureranging from 20 to 60 kgf/cm²G. The amount of the foaming agent to beused is suitably controlled according to the relationship between thefoamed particles intended to obtain, and a foaming temperature and/orthe quantity of heat at the high-temperature peak of the resultingexpansion-molded article.

No particular limitation is imposed on the dispersion medium used indispersing the resin particles therein so far as it does not dissolvethe resin particles. Examples of such a dispersion medium include water,ethylene glycol, glycerol, methanol and ethanol. Water is generallyused.

As the dispersing agent optionally added upon dispersing the resinparticles in the dispersion medium, there may be used fine particles ofaluminum oxide, titanium oxide, basic magnesium carbonate, basic zinccarbonate, calcium carbonate, kaolin, mica or clay. It is generallyadded to the dispersion medium in a proportion of 0.2 to 2 parts byweight per 100 parts by weight of the base resin.

The shock-absorbing material according to the present invention can beobtained by optionally subjecting the foamed particles thus obtained topretreatments such as application of an internal pressure to the foamedparticles, filling the foamed particles into a mold which can be closedbut not be sealed hermetically, heating the foamed particles with steamof about 3.0 to 6.0 kg/cm²G to secondarily expand the foamed particlesand at the same time mutually fusion-bond them, and then cooling amolded product thus obtained. In order to apply the internal pressure tothe foamed particles, the foamed particles are placed in a closedvessel, and pressurized air is fed to the vessel to increase theinternal pressure of the foamed particles to a predetermined pressure.

The density of the shock-absorbing material according to the presentinvention is generally 0.02 to 0.3 g/cm³, preferably 0.03 to 0.2 g/cm³from the viewpoints of sufficient energy absorption performance andlight-weight property.

Incidentally, the density of the expansion-molded article is determinedby finding a volume V (cm³) of the expansion-molded article from itsoutside dimensions and dividing the weight W (g) of the expansion-moldedarticle by this volume V (cm³).

Since the shock-absorbing material according to the present inventionhas high stiffness and excellent energy absorption efficiency, thesufficient stiffness and energy absorption efficiency can be retainedeven when the expansion ratio of the resulting molded article isenhanced to lighten its weight compared with the conventional products,or the thickness of the shock-absorbing material is thinned comparedwith the conventional products.

The shock-absorbing material according to the present invention istypically used as a core material for automotive bumpers. Besides, it ispreferably used as a automotive interior material integrated with a skinmaterial. Examples of the automotive interior material includedashboards, console lids, instrument panels, door panels, door trims,ceiling materials, interior materials for pillar parts, sun visors, armrests and head rests.

The present invention will hereinafter be described in more detail bythe following Examples (Examples 1 to 4) and Comparative Examples(Comparative Examples 1 to 3).

Various base resins having their corresponding melting points (°C.) andMI (g/10 min) shown in Table 1 were separately melted and kneaded in anextruder, and each of the thus-melted base resins was extruded into astrand through a die and quenched in water. The strand thus quenched waschopped into predetermined lengths, thereby forming pellets (weight:about 2 mg per pellet). The pellets (1,000 g) were dispersed in water(3,000 cc) in a closed vessel (volume: 5 liters). Dry ice (CO₂) wasadded in its corresponding amount shown in Table 1 to the dispersion,and kaolin (5 g) as a dispersing agent and sodiumdodecylbenzenesulfonate (0.05 g) as a surfactant were then added to thedispersion. While stirring the contents in the closed vessel, they wereheated to its corresponding holding temperature under heating shown inTable 1 without heating them to the melting completion temperature T_(e)of the base resin or higher and held for 15 minutes. The contents werethen heated to its corresponding foaming temperature shown in Table 1without heating them to the melting completion temperature T_(e) of thebase resin or higher and held for 15 minutes. Thereafter, pressurizednitrogen was introduced into the closed vessel to apply a back pressureof (the equilibrium vapor pressure of the foaming agent)+10 kg/cm²G tothe closed vessel. While keeping the back pressure, the vessel wasopened at one end thereof to release the resin particles and water atthe same time, thereby expanding the resin particles to obtain foamedparticles. The quantity of heat at the high-temperature peak and bulkdensity of the foamed particles thus obtained were measured. The resultsare shown in Table 1.

After the foamed particles were then dried for 24 hours in an ovencontrolled at 60° C., they were pressurized for 24 hours by pressurizedair of 2 kg/cm²G in a closed vessel to apply an internal pressure of 1.4kg/cm²G to the foamed particles. The thus-treated foamed particles werethen filled into a mold which can be closed but not be sealedhermetically and has a prescribed shape, and heated with steam under itscorresponding molding vapor pressure shown in Table 1 to mold them.After cooling the thus-obtained expansion-molded article, it was takenout of the mold and dried for 24 hours in an oven controlled at 60° C.to obtain a shock-absorbing material as a product.

The tensile modulus, quantity of heat at the high-temperature peak,density, compression stress and energy absorption efficiency of theresultant expansion-molded article (shock-absorbing material) weredetermined. The results are shown in Table 2.

The measuring methods of the quantity of heat at the high-temperaturepeak and bulk density of each foamed particle sample, and the measuringmethods of the tensile modulus, quantity of heat at the high-temperaturepeak, density, compression stress and energy absorption efficiency ofeach expansion-molded article sample are as follows.

(Measuring Method of Quantity of Heat at the High-temperature Peak ofFoamed Particles)

Such a DSC curve as shown in FIG. 1 was prepared by the differentialscanning calorimetry of each foamed particle sample to determine aquantity of heat corresponding to a section (an area hatched in FIG. 1)surrounded by a straight line connecting a point δ and a point β, astraight line connecting a point γ and the point δ, and a curveconnecting the point γ and the point β by calculation, said respectivepoints being shown in FIG. 1, and the numerical value thereof wasregarded as the quantity of heat at the high temperature peak.

(Measuring Method of Bulk Density of Foamed Particles)

The bulk density of each foamed particle sample was determined byproviding a container having a volume of 1,000 cm³ and an opening at itstop, filling the sample into the container at ordinary temperature andpressure, removing a portion of the foamed particle sample beyond theopening of the container, thereby substantially conforming the bulkheight of the foamed particle sample to the opening of the container,and dividing the weight (g) of the foamed particle sample within thecontainer at this time by 1,000 cm³.

(Measuring Method of Tensile Modulus of Expansion-molded Article).

A specimen was cut out of each expansion-molded article sample, andheated and pressed for 10 minutes by a hot press controlled to 220° C.,thereby melting and deaerating the specimen to form a sheet having athickness of 1 mm±0.1 mm. The sheet thus obtained was used to measureits tensile modulus in accordance with JIS K 7113 under the followingconditions:

Specimen: JIS No. 2 type Testing rate: 50 mm/min Distance betweenchucks: 80 mm.

(Measuring Method of Quantity of Heat at the High-temperature Peak ofExpansion-molded Article)

The quantity of heat at the high-temperature peak of eachexpansion-molded article sample was determined in the same manner as inthe measuring method of quantity of heat at the high-temperature peak ofeach foamed particle sample.

(Measuring Method of Density of Expansion-molded Article)

The density of each expansion-molded article sample was determined byfinding a volume V (cm³) of the sample from its outside dimensions anddividing the weight W (g) of the sample by this volume V (cm³).

(Measuring Method of Compression Stress and Energy Absorption Efficiencyof Expansion-molded Article)

A specimen 50 mm long, 50 mm wide and 25 mm high was cut out of eachexpansion-molded article (shock-absorbing material) sample, and thisspecimen was used to conduct a test under conditions of a specimentemperature of 20° C. and a rate of loading of 10 mm/min in accordancewith JIS Z 0234 Method A, thereby preparing a stress-strain diagram(illustrating an example thereof in FIG. 5) at the time a load wasapplied to the specimen on the basis of the test data. Compression (C)at the time of 50% strain was found from this diagram and regarded asthe compression stress of the expansion-molded article sample. On theother hand, the energy absorption efficiency of the expansion-moldedarticle sample was determined by the following equation:

 Energy absorption efficiency (%)=[(Area of OAB)/(Area of OABC)]×100

More specifically, the energy absorption efficiency (%) is expressed interms of a percentage of a value obtained by dividing an area surroundedby a line connecting points O, A and B in such a stress-strain diagramas illustrated in FIG. 5 by an area (area hatched in FIG. 5) of a squareregarding points O, A, B and C as vertexes.

The Compression stress reported in Table 2 of this application is astress at 50% strain found from the stress-strain diagram prepared byapplying a load to a specimen 50 mm long, 50 mm wide and 25 mm high asapparent from the description on pages 31 and 32 of the presentspecification and FIG. 5. The Energy absorption efficiency is also avalue at 50% strain.

On the other hand, the amount of the Unit volume energy absorption (E/A)described in column 12, lines 22 to 29 of Sugano et al. U.S. Pat. No.5,468,781 and in the Tables 2 and 3 of the patent is a value obtained bythe calculation of Stress (kgf/cm²) at 50% strain×Energy absorptionefficiency×0.5.

The compression stress in the present invention is determined in thesame manner as in Sugano et al. and corresponds to the stress at 50%strain in Sugano et al.

The Energy absorption efficiency shown in Table 2 of the specificationof the present application is expressed as a percentage obtained bymultiplying by a hundred a ratio of an area surrounded by a lineconnecting points O, A and B to an area of a square regarding points O,A, B and C at 50% strain in the stress-strain diagram as illustrated inFIG. 5. The Energy absorption efficiency in Sugano et al. corresponds toa ratio of an area surrounded by a line connecting points O, A and B toan area of a square regarding points O, A, B and C at 50% strain in thestress-strain diagram. Therefore, a value obtained by multiplying theEnergy absorption efficiency shown in Table 2 of the presentspecification by 0.01 corresponds to the Energy absorption efficiency inSugano et al.

Accordingly, a value obtained by multiplying the applicants' value ofCompression stress (kgf/cm²)×Energy absorption efficiency×0.01 by 0.5corresponds to the Sugano et al. amount of the unit volume energyabsorption (E/A).

The unit volume energy absorption E/A for applicants' Examples 1 to 5and Comparative Examples 1 to 3 are shown in Table 2 as amended.

It is understood from the above results that the shock-absorbingmaterials according to the present invention show higher compressionstress compared with the comparative expansion-molded articles and hencehave higher stiffness. In addition, the shock-absorbing materialsaccording to the present invention show higher numerical values even inenergy absorption efficiency than the comparative expansion-moldedarticles.

TABLE 1 Molding Foamed particles conditions Base resin Foamingconditions Quantity of Molding Melting Holding temp. Foamimg heat atBulk vapor MI point Amount of under heating temp. high-temp. densitypressure Kind of polymer (g/10 min) (° C.) CO₂ (g) (° C.) (° C.) peak(J/g) (g/cm³) (kg/cm²G) Ex. 1 Metallocene 15.7 150 90 145.5 150.5 36.50.056 3.6 propylene homopolymer Ex. 2 Metallocene ″ ″ 85 146.5 151.532.9 0.055 4.0 propylene homopolymer Ex. 3 Metallocene ″ ″ 80 146.0151.0 34.5 0.057 3.8 propylene homopolymer Ex. 4 Metallocene ″ ″ 75147.0 152.0 29.0 0.056 3.6 propylene homopolymer Ex. 5 Metallocene 30.1149 95 145.5 150.5 36.0 0.058 4.0 propylene homopolymer Comp.Metallocene 15.7 150 60 148.0 153.0 19.7 0.058 3.2 Ex. 1 propylenehomopolymer Comp. Metallocene ″ ″ 70 147.5 152.5 23.4 0.055 3.2 Ex. 2propylene homopolymer Comp. Metallocene ″ ″ 55 149.0 154.0 16.3 0.0533.0 Ex. 3 propylene homopolymer

TABLE 2 Quan- Unit Vol. Ten- tity of Energy Energy sile heat at absorp-Abs. AT Modu- high- Compress- tion 50% lus tem- ion effi- Strain (kgf/perature Density Stress ciency Kgf.cm/ cm²) peak (J/g) (g/cm³) (kgf/cm²)(%) cm³ Ex. 1 22,000 36.1 0.06 7.73 75.4 2.91 Ex. 2 ″ 31.9 ″ 7.28 72.42.64 Ex. 3 ″ 33.9 ″ 7.62 74.5 2.84 EX. 4 ″ 28.4 ″ 7.12 72.3 2.57 Ex. 520,000 35.8 ″ 7.30 73.8 2.69 Comp. 22,000 20.2 ″ 6.39 68.5 2.19 Ex. 1Comp. ″ 23.1 ″ 6.84 69.2 2.37 Ex. 2 Comp. ″ 15.4 ″ 5.50 68.3 1.88 Ex. 3

Industrial Applicability

The shock-absorbing materials according to the present invention havehigh stiffness and excellent energy absorption efficiency and thus areparticularly useful as bumper cores because they have all propertiesrequired of bumper cores when they are used as, for example, corematerials for automotive bumpers.

What is claimed is:
 1. A shock-absorbing material composed of anexpansion-molded article produced by using foamed particles comprising,as a base resin, only a polypropylene homopolymer obtained by using ametallocene polymerization catalyst or foamed particles comprising, as abase resin, a blend of a polypropylene homopolymer obtained by using ametallocene polymerization catalyst, as a main component, with anotherresin or elastomer, wherein the base resin has a tensile modulus of atleast 15,000 kgf/cm², the expansion-molded article has compressionstress of at least 7.28 kgf/cm² and an energy absorption efficiency of aleast 72.4 and has a crystal structure so that an inherent peak and ahigh-temperature peak appear as endothermic peaks on a DSC curveobtained by the differential scanning calorimetry of the molded article,said high-temperature peak meaning a peak appeared on the temperatureside higher than a temperature corresponding to the inherent peak ofendothermic peaks on a DSC curve obtained by heating 2 to 4 mg of aspecimen cut out of the expansion-molded article to 220° C. at a heatingrate of 10° C./min by means of a differential scanning calorimeter, anda quantity of heat at the high-temperature peak is at least 25 J/g. 2.The shock-absorbing material according to claim 1, wherein the baseresin is composed of only a polypropylene homopolymer obtained by usinga metallocene polymerization catalyst.
 3. The shock-absorbing materialaccording to claim 1, wherein the base resin is composed of a blend of apolypropylene homopolymer obtained by using a metallocene polymerizationcatalyst, as a main component, with another resin or elastomer.
 4. Theshock-absorbing material according to claim 1, wherein the base resinhas a tensile modulus of at least 15,500 kgf/cm².
 5. The shock-absorbingmaterial according to claim 1, wherein the quantity of heat at thehigh-temperature peak appearing on the DSC curve is at least 30 J/g. 6.The shock-absorbing material according to claim 1, wherein theexpansion-molded article has a crystal structure so that an inherentpeak and a high-temperature peak appear as endothermic peaks on a firstDSC curve obtained by heating 2 to 4 mg of a specimen cut out of theexpansion-molded article to 220° C. at a heating rate of 10° C./min bymeans of a differential scanning calorimeter, and only an inherent peakappears on a second DSC curve obtained by cooling the specimen from 220°C. to about 40° C. at a cooling rate of 10° C./min and heating it againto 220° C. at a heating rate of 10° C./min.
 7. The shock-absorbingmaterial according to claim 6, wherein a difference between thetemperature corresponding to the top of the high-temperature peakappearing on the first DSC curve and the temperature corresponding tothe top of the inherent peak appearing on the second DSC curve is atleast 5° C.
 8. The shock-absorbing material according to claim 1,wherein the density of the shock-absorbing material is 0.02 to 0.3g/cm³.
 9. The shock-absorbing material according to claim 1, wherein theshock-absorbing material is a core material for automotive bumpers. 10.The shock-absorbing material according to claim 1, wherein theshock-absorbing material is obtained by using foamed particles having abulk density of at least 0.04 g/cm³.
 11. The shock-absorbing materialaccording to claim 1, wherein the foamed particles are obtained byexpanding particles of the base resin, with an inorganic gas foamingagent.
 12. The shock-absorbing material according to claim 1, whereinthe expansion-molded article has compression stress of 7.28 to 7.73kgf/cm² and an energy absorption efficiency of 72.4 to 75.4%.
 13. Ashock-absorbing material composed of an expansion-molded articleproduced by using foamed particles comprising, as a base resin, only apolypropylene homopolymer obtained by using a metallocene polymerizationcatalyst or foamed particles comprising, as a base resin, a blend of apolypropylene homopolymer obtained by using a metallocene polymerizationcatalyst, as a main component, with another resin or elastomer, whereinthe base resin has a tensile modulus of at least 15,500 kgf/cm², theexpansion-molded article has a crystal structure so that an inherentpeak and a high-temperature peak appear as endothermic peaks on a DSCcurve obtained by the differential scanning calorimetry of the moldedarticle, said high-temperature peak meaning a peak appeared on thetemperature side higher than a temperature corresponding to the inherentpeak of endothermic peaks on a DSC curve obtained by heating 2 to 4 mgof a specimen cut out of the expansion-molded article to 220° C. at aheating rate of 10° C./min by means of a differential scanningcalorimeter, and a quantity of heat at the high-temperature peak is atleast 30 J/g, compression stress of at least 7.28 kgf/cm² and energyabsorption efficiency of at least 72.4%.
 14. The shock-absorbingmaterial according to claim 13, wherein the base resin is composed ofonly a polypropylene homopolymer obtained by using a metallocenepolymerization catalyst.
 15. The shock-absorbing material according toclaim 13, wherein the base resin is composed of a blend of apolypropylene homopolymer obtained by using a metallocene polymerizationcatalyst, as a main component, with another resin or elastomer.
 16. Theshock-absorbing material according to claim 13, wherein theexpansion-molded article has a crystal structure so that an inherentpeak and a high-temperature peak appear as endothermic peaks on a firstDSC curve obtained by heating 2 to 4 mg of a specimen cut out of theexpansion-molded article to 220° C. at a heating rate of 10° C./min bymeans of a differential scanning calorimeter, and only an inherent peakappears on a second DSC curve obtained by cooling the specimen from 220°C. to about 40° C. at a cooling rate of 10° C./min and heating it againto 220° C. at a heating rate of about 10° C./min, and wherein adifference between the temperature corresponding to the top of thehigh-temperature peak appearing on the first DSC curve and thetemperature corresponding to the top of the inherent peak appearing onthe second DSC curve is at least 5° C.
 17. The shock-absorbing materialaccording to claim 13, wherein the density of the shock-absorbingmaterial is 0.04 to 0.3 g/cm³.
 18. The shock-absorbing materialaccording to claim 13, wherein the expansion-molded article hascompression stress of 7.28 to 7.73 kgf/cm² and an energy absorptionefficiency of 72.4 to 75.4.